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alt="squared"/> hybrid bonds enclosing angles of 120Superscript ring. The parent material of graphene is graphite, which consists of a stack of graphene sheets that are weakly bonded to each other. In fact, graphene can be isolated from graphite by peeling off flakes with a piece of scotch tape. In diamond, the carbon atoms form spcubed‐type bonds and each atom has four nearest neighbors in a tetrahedral configuration. Interestingly, the diamond structure can also be described as an fcc Bravais lattice with a basis of two atoms.

Schematic illustration of structures for (a) graphene, (b) graphite, and (c) diamond. Bonds from sp2 and sp3 orbitals are displayed as solid lines.

      After having described different crystal structures, the question is of course how to determine these structures in the first place. By far the most important technique for this is X‐ray diffraction. In fact, the importance of this technique extends far beyond solid state physics, as it has become an essential tool for fields such as structural biology as well. In biology, the idea is that you can derive the structure of a given protein by trying to crystallize it and then use the powerful methodology of X‐ray diffraction to determine its structure. In addition, we will also use X‐ray diffraction as a motivation to extend our formal description of structures.

      1.3.1 X‐Ray Diffraction

      X‐rays interact rather weakly with matter. A description of X‐ray diffraction can therefore be restricted to single scattering, meaning that we limit our analysis to the case that X‐rays incident upon a crystal sample get scattered not more than once (most are not scattered at all). This is called the kinematic approximation; it greatly simplifies matters and is used throughout the treatment in this book. Furthermore, we will assume that the X‐ray source and detector are placed very far away from the sample so that the incoming and outgoing waves can be treated as plane waves. X‐ray diffraction of crystals was discovered and described by M. von Laue in 1912. Also in 1912, W. L. Bragg came up with an alternative description that is considerably simpler and will serve as a starting point for our analysis.

      1.3.1.1 Bragg Theory

      Bragg treated the problem as the reflection of the incident X‐rays at flat crystal planes. These planes could, for example, be the close‐packed planes making up fcc and hcp crystals, or they could be alternating Cs and Cl planes making up the CsCl structure. At first glance, the physical justification for this picture seems somewhat dubious, because the crystal planes appear certainly not “flat” for X‐rays with wavelengths on the order of atomic spacing. Nevertheless, the description proved highly successful, and we shall later see that it is actually a special case of the more complex Laue description of X‐ray diffraction.

      It is obvious that if this condition is fulfilled for one specific layer and the layer below it, then it will also be fulfilled for any number of layers with identical spacing. In fact, the X‐rays penetrate very deeply into the crystal so that thousands of layers contribute to the reflection. This results in very sharp maxima in the diffracted intensity, similar to the situation for an optical grating with many lines. The Bragg condition can obviously only be fulfilled for lamda less-than 2 d, putting an upper limit on the wavelength of the X‐rays that can be used for crystal structure determination.

Schematic illustration of construction for the derivation of the Bragg condition. The horizontal lines represent the crystal lattice planes that are separated by a distance d. The heavy lines represent the X-rays.

      1.3.1.2 Lattice Planes and Miller Indices

Schematic illustration of three different lattice planes in the simple cubic structure characterized by their Miller indices.

      1 We find the intercepts of the specific plane at hand with the crystallographic

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